The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential.
In a four-wheel drive motor vehicle, a conventional torque distribution ratio of front wheels and rear wheels is determined in accordance with a dynamic load ratio of the front and rear wheels, considering the movement of the center of gravity of the vehicle at acceleration so that the power of the engine may be most effectively used.
In the four-wheel drive vehicle based on mainly front-drive with a front-mounted engine, the ratio of front torque T.sub.F to the front wheels and rear torque T.sub.R to the rear wheels are in the ratio of 50:50. In the four-wheel drive vehicle based on mainly rear-drive with a front-mounted engine, the front torque T.sub.F and the rear torque T.sub.R are in the ratio of 40:60. A central differential comprising bevel gears is provided in the former system in which the output torque of the engine is equally distributes, and a central differential having a simple planetary gear device is provided in the latter system.
The former system ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driveability of the vehicle is improved. However, when the vehicle a turn at high speed under the differential lock all of four wheels may slip (i.e. the vehicle at the same time, causing difficult driving.
In order to ensure a driving stability of the vehicle, the torque to the rear wheels is set to a larger than that to the front wheels by arranging the differential comprising the simple planetary gear device, so that first the rear wheels may slip. Thus, the vehicle can be safely driven by the front wheels at a small torque while the rear wheels do not drive.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. The output of a transmission is transmitted to a carrier of the planetary gear device. The torque is distributed to front wheels through either a sun gear or a ring gear and to rear wheels through the other gear. The torque to front and rear wheels is unequally distributed at the ratio determined by the difference between the pitch circles of the sun gear and the ring gear. A fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation. A wide range of torque distribution ratio determined by the ratio of the pitch circle can not be attained because the diameters of the sun gear and the ring gear are limited. However, it is difficult to change the diameters because of limitation of space.
Further, in a power transmitting system where an output shaft of a transmission is inserted in the sun gear of the planetary gear device, the diameter of the sun gear becomes large, which renders the size of a transfer device large and hence the space in the vehicle is reduced. Since the distribution of torque to the rear wheels can not be set to a large value, the control range of the multiple-disk friction clutch becomes small. Consequently, a motor vehicle having good driveability and, controllability can not be provided.